1. Field of the Invention
The present invention relates to a green coherent light generating device by means of second harmonic generation (SHG) using a semiconductor laser and KTP crystals, and a method for generating the green coherent light.
2. Description of the Related Art
Green coherent light is used in various fields, for example, optical displays, image related devices, a pump beam of an optical parametric oscillator, and so on. Up to the present, green light is obtained using SHG light of a YAG laser or an Argon laser, and so on. But these lasers need large-scale devices and have the problem that quality of the green light is not good. So a technology to generate coherent green light combining a semiconductor laser and a KTP crystal was developed. (For example, refer to the pages 1192 of the 50th meeting drafts, The Japan Society of Applied Physics and Related Societies (following non-patenting reference 1).)
FIG. 1 shows a schematic diagram of a conventional device (hereinafter referred to as a “conventional device”) that generates coherent green light by combining a semiconductor laser and a KTP crystal. As shown in FIG. 1, a conventional device 1 comprises a semiconductor laser 2 which generates 1080 nm wavelength light, an optical resonator 3 into which the light from a semiconductor laser is injected, and one a-cut KTP crystal 4 provided in the optical path in the optical resonator. Two concave mirrors, for example, 3a and 3b compose the optical resonator (optical cavity) 3, and these mirrors may face each other. In addition, although not illustrated especially, another example of the conventional device is a device using a ring-type optical resonator.
A laser beam which has the wavelength of 1080 nm output from the semiconductor laser 2 is introduced into the optical resonator 3 through mirror 3a. It is built up in the optical resonator, the intensity increases, and the light introduced into the optical resonator generates SHG light by the nonlinear effect in the KTP crystal. In order to generate SHG efficiently at the above-mentioned wavelength, the phase matching called TYPE II is taken, and the ordinary and extraordinary rays are used in the KTP crystal that has refractive-index anisotropy. By the refractive-index anisotropy of the KTP crystal, for example, since horizontal polarization and perpendicular polarization have different refractive indices of the KTP crystal, the ordinary and extraordinary rays will experience a different optical path length within a KTP crystal. Moreover, light with a wavelength of 1080 nm which is confined and built up within the optical resonator generates light with a wavelength of 540 nm as SHG light by the nonlinear effect in the KTP crystal. This SHG light is output from the output mirror of the optical resonator.
As shown in FIG. 1, the refractive index of the polarization of the ordinary ray is no within a KTP crystal, and the optical path length of the ordinary ray within the KTP crystal is no1, provided that the length of the KTP crystal is set to 1. On the other hand, the refractive index of the polarization of the extraordinary ray is ne, and the optical path length of the extraordinary ray within the KTP crystal is ne1.
In order for light to resonate within the optical resonator and to obtain a powerful output, standing waves must be made within the optical resonator. Namely, a powerful output is obtained by a resonance phenomenon when the optical length is the integral multiple of half-wavelength (however, in a ring-type resonator, it resonates at the time of the integral multiple of wavelength.). If the wavelength of the laser beam from a semiconductor laser is set to λ, and the optical path length in the optical resonator which does not have a KTP crystal is set to L, by making m1 and m2 into an integer, the resonance conditions for the horizontal and perpendicular polarizations are respectively m1 λ/2=L+(no−1)1 and m2 λ/2=L+(ne−1)1. Since KTP crystals have refractive-index anisotropy, no differs from ne. Therefore, in order to have fulfilled the above-mentioned resonance conditions, L should be adjusted, and also the refractive index of the KTP crystal needed to be controlled by carefully adjusting the crystal temperature.
FIG. 2 is a graph that shows the relation between the resonator (cavity) length and resonance in the case of changing temperature using the conventional device. FIG. 2 shows that in a certain conventional device, the optical resonator length at which the ordinary ray resonates and the optical resonator length at which the extraordinary ray resonates correspond when the temperature of the crystal is 66.6° C. Therefore, at 66.6° C., if the optical resonator of prescribed length is adopted, a resonance phenomenon will happen. However, if the temperature is far from 66.6° C., the resonator length at which the ordinary ray resonates and the resonator length at which the extraordinary ray resonates do not correspond. A permitted range of the temperature is considered to be about 1/100° C. or less. As shown in FIG. 2, resonance conditions do not meet at 64.6° C. and 68.6° C., which are 2° C. away from optimal temperature 66.6° C. Furthermore, at 62.6° C. and 70.6° C., which are 4° C. away from optimal temperature, since the resonator length at which the ordinary ray resonates and the resonator length at which the extraordinary ray resonates are completely different, the resonance cannot be obtained simultaneously. Therefore, in the conventional device, in case of obtaining resonance simultaneously with the ordinary and extraordinary rays, there was a problem that the temperature of the KTP crystal had to be precisely controlled.
FIG. 3 is a graph that shows the relation between the SHG light output of the laser beam only by the nonlinear crystal without an optical resonator, and the SHG light output of the laser beam by the conventional device having an a-cut KTP crystal in an optical resonator. In FIG. 3, a dotted line is the SHG light output of the laser beam only by the nonlinear crystal, and circles are the SHG light output of the laser beam obtained by placing an a-cut KTP crystal in the optical resonator. As explained previously, if the nonlinear crystal is placed in the optical resonator, only at specific temperatures, resonance phenomena will occur simultaneously with the ordinary and extraordinary rays, powerful SHG light will be obtained, and SHG light output will not be obtained except at the specific temperatures.
FIG. 3 shows that, for example, although the maximum output of the SHG (single path) of an a-cut KTP crystal unit is obtained at about 62° C., even if it is going to obtain green coherent light using a conventional device, the resonance does not occur at 62° C., which is the temperature that gives the maximum output of SHG light. On the other hand, FIG. 3 shows that, since the ordinary and extraordinary rays resonate at about 39° C., about 52° C., and about 67° C., SHG light output is obtained from the optical resonator comprising the above-mentioned a-cut KTP crystal. However, these temperatures differ from the temperature at which the maximum efficiency of the nonlinear crystal itself is acquired (the above-mentioned a-cut KTP crystal is about 62° C.). Therefore, in case of obtaining SHG light using the conventional device, there was a problem that the output of SHG light did not become large efficiently. Moreover, there was also a problem that if the temperature of a crystal was not stabilized within about 1/100° C. or less, stable SHG light output was not obtained.
[Non-patenting reference 1] Page 1192 of the 50th meeting drafts, The Japan Society of Applied Physics and Related Societies